Continuous constant pressure staging of solid-vapor compound reactors
BACKGROUND OF THE INVENTION 
     The use of compounds comprising solid-vapor compositions formed by adsorption of gas molecules on a solid adsorbent as heat pump working materials is known in the art. Heat pump systems using such materials have a number of advantages over other heat pumps for residential and commercial space conditioning, industrial heat pumping and refrigeration. Such advantages include higher temperature lift created by the solid-vapor media as compared to other sorption media thus eliminating the need for cooling towers or lift staging. Moreover, the apparatus used for the solid-vapor compound heat pumps require few, if any, moving parts, resulting in simple and reliable hardware. Additionally, such systems do not use the objectionable CFC&#39;s. 
     The solid-vapor compounds suitable for heat pumps include complex compounds which are materials which adsorb molecules of gas to form coordinative bonds in which the gaseous reactant coordinates via electron displacement with the solid adsorbent, commonly a solid metal inorganic salt. The adsorption/desorption process releases significant heat during adsorption and adsorbs energy during the desorption phase. Unlike most other sorption processes, the entire adsorption or desorption reactions may occur at constant temperature thus eliminating problems with hot and cold sorber ends. Useful gaseous reactants include water, ammonia, methanol, methane, ethane and the like. A number of such materials are described in co-pending applications Ser. Nos. 115,820, filed Nov. 2, 1987 and 162,016, filed Feb. 29, 1988 now U.S. Pat. Nos. 4,822,391 and 4,848,994, respectively. Such compounds and their uses described in the aforesaid co-pending applications are incorporated herein by reference. 
     Heat activated heat pumps consist of a heat engine subsystem which generates high pressure refrigerant vapor, essentially a thermal compressor, and a heat pump subsystem which uses high pressure refrigerant to produce cooling or heat pumping. The thermal compressor, heat pump, and their combination in a heat activated heat pump comprise useful thermodynamic systems which make advantageous use of solid-gas reactions. It is an object of the present invention to use such reactions to even greater advantage and efficiency. Moreover, thermal energy and cool storage systems may also be improved by using staging techniques of the present invention with respect to charge and discharge temperatures as well as energy density. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a system utilizing methods and apparatus designed for making highly advantageous use of solid-vapor adsorption/ desorption technology. In the reactions, solid reactants react with gaseous reactants to form compounds in which the gas is alternatively adsorbed and desorbed. In the process of the invention, a plurality or series of different compounds are selected based on the vapor pressure of the gaseous reactant. Utilizing a plurality of reactors each of which is charged with a plurality of two or more different solid reactants, the materials are made to adsorb or desorb gaseous reactant at a given constant pressure by adjusting the pressure below or above the equilibrium vapor pressure of the gas. Each of the compounds of a plurality or set of compounds used in the reactors has a different gaseous reactant vapor pressure which is substantially independent of the concentration of the gaseous reactant in the reactor. By selecting the appropriate solids in the set of compounds used to charge the different reactors in the apparatus, and by selecting an appropriate pressure for desorption reactions and typically a different pressure for adsorption reactions, the system can be made to function to take full advantage of continuous adsorption and desorption of the different compounds to achieve thermal compression, heat pumping through mechanical or thermal activation and thermal energy storage. The system of the invention achieves improved efficiency using relatively simple hardware. These as well as other advantages will be evident from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustrating and example of an apparatus used in the system of the invention; 
     FIG. 2 is a phase diagram illustrating a preferred embodiment the process of the invention carried out in the apparatus illustrated in FIG. 1; and 
     FIG. 3 illustrates a mechanically activated heat pump apparatus embodiment which may be used in a system of the invention. 
    
    
     DETAILED DESCRIPTION 
     Heat Activated Heat Pump 
     As used herein, the term &#34;compound&#34; is intended to mean any reaction product formed by adsorption and desorption of a gaseous reactant on a solid reactant within the scope of the invention. In practicing the continuous staging of a constant pressure engine cycle according to the invention, a plurality of two or more different solid reactants are selected, and a plurality or set of different solid reactants is introduced into each reactor in the heat pump apparatus. Each of the compounds of such sets or groups each exhibit different vapor pressure curves, i.e., each has a different vapor pressure-temperature relationship, and which is independent of the concentration of the gaseous reactant. Thus, each of the compounds in a set in a reactor adsorb and desorb the same gaseous reactant at a different temperature at the reaction pressure in the reactor. Compounds are selected and arranged in the reactor in sequence of ascending order of gas vapor pressure. Preferably the compounds of the series are selected so that none of the compounds in the same reactor have an additional coordination step at lower equilibrium temperature which may adsorb more reactant gas from the other compounds during temperature equilibrium or shut-down condition which would reduce cycle performance during intermittent operation. Moreover, masses of each compound are adjusted so that the amount of heat required to desorb each compound is proportional to the temperature difference between that compound and the next higher temperature compound. 
     The compounds are arranged in the reactors in sequence based on the compound gaseous vapor pressure, and preferably are arranged successively in ascending order of gas vapor pressure. The reactors are provided with means for directing a heat transfer fluid to thermally communicate with the compounds. During process operation the heat transfer fluid is gradually cooled as it passes through a desorbing reactor in which the successive compounds desorb the gaseous reactant at successively lower temperatures. In the adsorbing reactor, the fluid will become gradually heated as it is successively exposed thermally to the succession of adsorbing compounds in which next successive compound in the sequence adsorbs at a higher temperature. 
     Specific reactants used to form compounds useful in the invention include metal oxides, hydrides, halides, carbonates, nitrites, nitrates, oxalates, sulfides and sulfates. Preferred metals for the inorganic salts are selected from alkali and alkaline earth metals, transition metals, aluminum, zinc, cadmium and tin. Preferred transition metals are manganese, iron, nickel, and cobalt. Double metal chloride salts of metals selected from alkali metals, alkaline earth metals, aluminum, manganese, iron, nickel and cobalt are also useful. Hereinafter these reactants will be sometimes referred to as solids, salts or solid reactants. 
     Gaseous reactants which are adsorbed on the solids to form compounds which are especially useful in the processes of the invention are ammonia, water, methyl amine and methanol, ammonia being especially suitable because it is stable, and forms high energy complexes. However, hydrogen, carbon dioxide, sulfur dioxide, other lower alkanols, lower alkanes, particularly methane and ethane, pyridine, alkylamines, polyamines and phosphine may also be used. These gaseous reactants may also be referred to as refrigerants herein. 
     In a specific example of a set or series of compounds, to illustrate a system according to the invention, salts MgBr 2 , CoBr 2 , CoCl 2 , CaBr 2  and SrBr 2  are used in a heat pump consisting of two separate reaction vessels. The compounds comprise the ammonia ligand complex compound of the aforesaid salts with the MgBr 2 , CoBr 2 , CoCl 2  and CaBr 2  salts forming complexes containing 2 to 6 NH 3  and SrBr 2  containing 2 to 8 NH 3 . FIG. 1 illustrates schematically an example of an apparatus embodiment for carrying out the continuous constant pressure staged heat pump with the compounds designated A-E respectively in the order given above beginning with MgBr 2  . XNH 3 . The salts are charged to reactors 10 and 20, in successive ascending order of the complex compound ligand vapor pressure. Thus, the set of salts in each reactor is aligned as shown successively A-E. In each reactor, there is provided a conduit or equivalent means for supplying a heat transfer fluid to thermally contact the compounds. The compounds may be present in a column in the order as shown, with the transfer fluid supply means comprising a pipe and having suitable means to exchange heat with the compounds. The apparatus includes a burner or furnace 15 with conduits 26, 28, 38 and 42 which direct the heat transfer fluid between furnace 15, reactors 10 and 20, and heat exchanger 25. A valve 22 and pump 18 provide means to assist in directing the heat transfer fluid through the system. Evaporator 30 and condenser 32 are also connected with the reactors via pipes 36 and 37 and valve 34 for directing ammonia vapor to and from the reactors. 
     In a first reaction phase or half-cycle, valve 22 is positioned such that hot heat transfer fluid is directed via conduit 26 into reactor 10. With the compounds arranged according to their ascending order of vapor pressure the heat transfer fluid will successively thermally communicate with the compounds in the set as it travels along the length of reactor 10. 
     In this reaction cycle, reactor 10 is the desorption reactor while reactor 20 is the adsorption reactor. Reactor 10 is pressurized to a first pressure, while reactor 20 is pressurized to a second pressure, lower than the first pressure. The desorption reactions in reactor 10 are driven by the heated heat transfer fluid introduced into the reactor via pipe 26 thereby driving these desorption reactions, successively, whereby the heat transfer fluid is gradually cooled as it gives up heat to the desorbing compounds. The cooled heat transfer fluid is then directed via conduit 28 through heat exchanger 25 where it is further cooled to a temperature suitable for introduction into reactor 20 via conduit 38. Reactor 20, in this phase or half-cycle of the process, is the adsorbing reactor in which the set of compounds therein adsorb the gaseous reactant in exothermic reactions. In this reactor, the heat transfer fluid is gradually heated as it is directed along the reactor and is successively exposed thermally to the exothermic adsorption reactions at successively higher temperatures. Thus, as the heat transfer fluid leaves reactor 20 via pipe 42, it is heated substantially relative to the temperature at which it was introduced via pipe 38. The heat transfer fluid is then directed back to furnace 15 where it is again heated to the temperature necessary for driving the endothermic reactions in reactor 10. 
     During this cycle of the process, the gaseous reactant from the desorption reactor 10 is directed to the condenser 32, and gaseous reactant for the adsorption reactions in reactor 20 is obtained from evaporator 24. The evaporator and condenser are in thermal contact with heat exchangers, not shown for transferring and recovering energy to and from the gaseous reactant. 
     In the second half-cycle or phase of the process, the pressure in the reactors is reversed such that reactor 20 becomes the desorbing reactor with reactor 10 being the adsorption reactor. Valve 22 is adjusted so that the heated heat transfer fluid is directed initially via pipe 42 to reactor 20, with the reactions then occurring as previously described in the first reaction phase but with the reactors reversed for adsorption and desorption. At the conclusion of the second half-cycle, the valves are again reversed and the first half-cycle as above described repeated. 
     By way of example, the previously described compounds charged into the reactors adsorb and desorb at temperatures and at pressures indicated in the following table. A phase diagram of a process carried using these or similar compounds is illustrated in FIG. 2. For example, in the first half-cycle as previously described, the desorption reaction in reactor 10 will be carried out at substantially constant pressure between points (1) and (3) illustrated in the diagram. The gradual temperature reduction of the heat transfer fluid is also indicated. Additional heat is rejected or removed from the heat transfer fluid between points (2) and (3) through heat exchanger 25 shown in FIG. 1. Similarly, adsorptions in reactor 20 are carried out at the lower pressure between state points (3) and (4). The heat transfer fluid is then heated to the elevated temperature indicated between points (4) and (5), and the cycle continued. The process is reversed when desired or when necessitated by the extent of adsorption and desorption of the compounds in that cycle phase. 
     
                       TABLE I                                                     
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             Desorb  Adsorb                                               
             (7.31 Bar)                                                   
                     (1.43 Bar)                                           
NH.sub.3       Temp. °K.                                           
______________________________________                                    
MgBr.sub.2.2/6 562       502                                              
CoBr.sub.2.2/6 514       456                                              
CoCl.sub.2.2/6 461       416                                              
CaBr.sub.2.2/6 416       368                                              
SrBr.sub.2.2/8 405       360                                              
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     From this example of a set or series of complex compounds it is shown that the salts are arranged in decending order of absorption and desorption temperature, which corresponds to an ascending order of compound vapor pressures. 
     If desired, the evaporator and condenser shown in FIG. 1 may be replaced by reactors containing a solid salt that will adsorb and desorb the gaseous reactant, for example, sodium bromide. Such reactors, cooperating with heat exchangers, alternatively provide heating and cooling. During each half-cycle, one reactor provides cooling while the other is heating. The advantages of using the sodium bromide, or other solid reactant, for adsorbing/desorbing the gaseous reactant as opposed to the condenser/evaporator equipment include (1) higher energy density in the adsorption/desorption reactions as compared to evaporation and condensation of the gas, resulting in higher coefficients of performance and less system mass, and (2) the ability to reject heat at high temperature with lower system pressure than would be required using refrigerant condensation. 
     Thermal Compressor 
     The process and system of the invention may also be used as a thermal compressor, which is a subsystem of a heat activated heat pump. For example referring to FIG. 1, by removing condenser 32 and evaporator 30, the resulting sub-system apparatus acts as a thermally activated compressor which receives low pressure vapor through conduit 38 and delivers high pressure vapor through conduit 39. Such a thermal compressor may be used as a less expensive alternative to an electrically driven compressor for obtaining pressurized gaseous reactants. For example, the high pressure gas, from the high pressure desorption reaction vessel, may be directed to a gas turbine, or other apparatus having means for using the gas to perform mechanical work. The resulting lower pressure gas (gaseous reactant) is then returned to the low pressure reactor. Constant pressure staging of appropriate compounds can provide efficient thermal compression of any gaseous reactant, such as water, ammonia, carbon dioxide, sulfur dioxide, methanol and other lower alkanols, alkylamines, polyamines, and phosphine. Hydrogen can also be compressed by use of hydrides in the reactors in place of complex compounds while carbon dioxide can be used with metal oxide/metal carbonate reactants, and water can be used with metal oxide/metal hydroxide or complex compound reactants. Such thermal compression using a process of the present invention is more efficient than conventional thermal compressors because heat is cascaded through several stages and high pressure vapor is generated at each stage. 
     Heat Pumps Activated by Mechanical Work or Pressure 
     The constant pressure staging process of the invention may also be used in a heat pump which receives high pressure refrigerant vapor, discharges low pressure vapor, and produces cooling or heating. The high pressure refrigerant vapor can be provided by a mechanical compressor, thermal compressor, for example a constant pressure staged thermal compressor described above, or other source. The constant pressure staged heat pump is most advantageous when coupled with a mechanical compressor, because efficiency and reliability of mechanical compressors increase as compression ratio is decreased and the staging lowers the compression ratio for a given temperature lift or increases the temperature lift for a given compression ratio. Such an apparatus is illustrated in FIG. 3, which is quite similar to the apparatus shown in FIG. 1 except that the evaporator, condenser and furnace or burner have been deleted. In place of the evaporator and condenser, a compressor 40 is used for providing high pressure refrigerant vapor to the reactors in cooperation with the conduits to and from the reactors and valve 34 for directing the gaseous reactant to and from the reactors. Heat exchangers 25 and 27 are used to remove or introduce heat or energy to and from the heat transfer fluid passing successively between the reactors. Again, the compounds are introduced in a set of compounds in ascending order of complex compound vapor pressure. In using mechanical compressor means of FIG. 3 for driving the reactions, the pressures in the vessels described in FIG. 1 are simply reversed.